Spin on glasses (SOGs) are known materials used in the fabrication of semiconductor devices. SOGs are primarily used to planarize a semiconductor wafer, but are also used as inter-layer dielectrics, passivation layers, plug regions, and sacrificial etch-back layers. As the name "spin on glasses" suggests, SOGs are deposited onto a semiconductor substrate and the substrate is spun at high speeds to distribute the SOG uniformly across the substrate surface. SOGs are initially in a liquid state, which allows the SOG to fill any open spaces, contacts, crevices, or voids which might be on a substrate surface. The coated substrate is then baked to remove residual water and solvents and to densify the SOG into a solid, glass-like film.
Conventional SOGs can be classified into two general categories, silicate SOGs and siloxane SOGs. Silicate SOGs are polymer networks containing primarily Si--O bonds. Silicate SOGs have an advantage of forming highly pure silicon dioxide (SiO.sub.2) films, without organic contaminants. A disadvantage associated with conventional SiO.sub.2 SOGs is that upon baking, the films shrink considerably, creating high stresses in the films and in an underlying substrate. Another disadvantage is that SiO.sub.2 SOGs do not planarize uniformly or sufficiently. For example, a silicate SOG may planarize a small space between two structures, but as the size of the space increases the planarization capability is reduced. In general, SiO.sub.2 SOGs reduced topography when compared to chemical vapor deposited (CVD) conformal films, but also created undesirable mechanical stress and strain in integrated circuits.
In addition, SiO.sub.2, due to mechanical and chemical properties, could not fill very narrow trenches or contacts. When CVD SiO.sub.2 is used to form plugs in narrow trenches or trenches with a high aspect ratio, a known and undesirable phenomenon called crevice formation occurs. The compatibility of materials with narrow trenches or contacts is important because narrow trenches and contacts are becoming widely used as integrated circuit cell sizes decrease and photolithographic minimum geometry sizes shrink. Another disadvantage related to the use of SiO.sub.2 is that SiO.sub.2 formations and layers cannot be planarized via conventional reflow techniques and therefore are less planar than currently desired.
In an effort to improve planarization capabilities of SOGs, semiconductor manufacturers have used several techniques. One technique used is to apply a very thick SOG film due to the fact that thick SOG films can be made more planar than thin SOG films. However, very thick SOG films have a greater tendency to crack, particularly upon baking. Another technique is to apply multiple, thin SOG films to improve planarity without experiencing cracking problems. While several thin films can improve planarity with greater resistance to cracking, using multiple films significantly increases manufacturing time. Another way to improve planarization capability of a SOG is to add dopants, such as boron or phosphorus, to the film. These dopants aid in the reflow of the SOG during subsequent anneal, and improve the planarity of the SOG layer. However, there are applications in semiconductor devices in which doping an SOG is not appropriate, and the doped SOGs still results in stress and strain above desired levels.